This invention relates to formation resistivity measuring instruments and more specifically to improvements in the propagation properties of the antennas. A unique and improved design of the antenna radiating groove on the mandrel of resistivity tool is disclosed.
In measurement-while-drilling (MWD) operations, the resistivity measuring instrument is included in the drill string while the oil or gas borehole is being drilled. The resistivity sub usually has a plurality of electromagnetic coils spaced longitudinally of the tool mandrel. The antenna coils cannot be wound on the surface of the mandrel because of the hostile abrasive environment in the borehole during drilling in which the antenna coils would be damaged. The coils cannot be covered with an electrical conducting metal to act as an abrasion shield because such conducting material would also inhibit the electromagnetic propagation from the antenna into the formation. Accordingly, the electromagnetic radiating coils have been recessed in circumferential grooves in the tool mandrel and the grooves filled with a non-conducting and abrasion resistant material such as a selected epoxy resin.
However, the mechanical bond of the epoxy resin material often fails with respect to the antenna groove and the material is radially displaced outwardly from the groove which only hastens further abrasion and failure. In addition, while it is desired to maximize the surface area of the antenna slot to maximize the electromagnetic propagation from the antenna coil leaving the mandrel, increasing the width of the coil usually only magnifies the bonding failure and abrasion problems as above described.
Accordingly, one feature of the present invention is to provide an improved antenna groove design that will enhance the surface area of the groove for maximizing electromagnetic propagation while minimizing mechanical failure of the annular ring of non-conducting material with respect to the groove.
Another feature of the present invention is to provide an improved antenna groove configuration that will maximize the surface area for bonding between the non-conducting material and the groove sidewalls.